The use of a laser beam for scanning an original and/or recording information on a laser sensitive medium has been disclosed in the prior art as shown, for example, in U.S. Pat. No. 3,946,150. For recording purposes, a laser beam is directed to a modulator which modulates the laser beam in accordance with information to be printed on the laser sensitive medium, such as a charged xerographic surface. The modulator beam is directed to a rotating optical scanner, driven by a motor, which acts to scan the beam across the xerographic surface.
While for relatively low-accuracy requirements the motor driving the rotating scanner can be made to function adequately without speed sensing and feedback control, this generally can only be accomplished by careful and perhaps expensive attention to the elimination of all sources of disturbances of the motor.
It has been found that for accurate speed control at relatively high speeds a hysteresis synchronous motor would be preferable to drive the rotating scanner. However, these motors tend to exhibit characteristic speed variations called "hunting" which is a rotational oscillation about its equilibrium or steady state velocity and which will have adverse effects in scanning systems which use hysteresis synchronous motors. The angle between the mechanical position of the motor rotor and the electrical position of the rotating electric field constitutes a phase lag. A disturbance to the motor shaft can cause hunting because the lag angle is oscillating about its steady state value. If the drag (damping) component of the motor load could be accurately controlled so as to provide critical damping or slight overdamping no oscillation would occur. However, sources of friction are difficult to control and may slow the initial acceleration of the motor and even cause the motor to run below its synchronous speed. Hunting is a short term speed variation of the motor, wherein the average speed remains constant and the instantaneous speed undergoes periodic excursions above and below the average. Thus, an alternative is to provide a speed control system whose response time is rapid enough to control these short term variations without reducing the available motor torque or acceleration.
The prior art discloses various systems for controlling a synchronous motor in a manner to eliminate or substantially reduce the huntingcharacteristic. For example, in U.S. Pat. No. 3,609,488 a motor speed control system is disclosed. However, the system disclosed therein does not provide the precise speed control desired in high resolution and close registration tolerance optical scanning systems. Copending application Ser. No. 756,511 filed Jan. 3, 1977 and assigned to the assignee of the present invention, discloses a precise speed control system for controlling a synchronous motor used to drive an optical scanner motor. In this copending application the amplitude of the signals utilized to drive the motor field windings are modulated in a manner such that the motor torque is adjusted in response to the modulated drive signal such that hunting in the motor is damped out. Although the system described in the aforementioned copending application is an effective way of damping motor hunting, alternate motor speed control techniques for damping motor hunting would be desirable.